I'm not so sure I'm ready to condemn the passenger just yet. She found a car-pooling arrangement which worked for her, because it meant her husband could have the van during the day, and she compensated the OP with gas money. Since the OP didn't state that the arrangement wasn't working for her, she could only assume it was win-win all around (OP gets help with gas money which she would have had to pay for alone, coworker gets ride to work without having to pull the van into action).
She did say, "Oh it must be inconvenient for you" thereby checking with the OP to see if she was SURE it was OK. The OP seemed not to mind at all. So she continued with the rides, assuming that all was well and the OP was happy not to have to shoulder the gas burden alone.
However, since the OP had secretly been looking forward to this woman's contract ending, and that didn't happen, it's definitely time to put a stop to the rides (personally, I would loathe going 15 minutes out of my way every day, and I would loathe having to chat in the car too). But I'm not so ready to say the coworker has done anything wrong, since she didn't know the OP's feelings on the matter.
How she reacts to being told her rides are ending will be the true test of her character.